How to see several texture slots at once? (cycles)
To see the effect of a new image added in the material draw mode when painting, be sure to actually have it mixed into your other image as if you were dealing with layers in an image editor program. Here I use the image of alpha 1 and white as a base and another image with 0 alpha to overlay on top of the white image, mixed with factor of 1 in the color mix node before sending to the shader. I plug the alpha socket of the 0 alpha image into the mix factor of the color mix node to control how it mixes them, revealing the colors underneath in the other image.

 Remember to keep adding the images to the lower mix inputs, as Blender uses a top to bottom stack (like added lines of code in a text editor)versus the bottom to top style we see in image editors(like layers of acetate on top of each other). I believe the disconnected image node is mainly used for baking out a shader to a single image.